Properties of Materials

1. Properties of Materials DHYG 113 Restorative Dentistry I

2. Objectives • Describe the physical, chemical, biologic, and mechanical properties of commonly used dental materials • Explain the application of properties in the use of materials to insulate pulp • Define coefficient of thermal expansion • Define viscosity and wetting and apply to the science of dental materials • Explain how hardness is measured and relate to biocompatibility of various materials

3. Objectives • Define modulus of elasticity and its relationship to stress and strain • Explain how plastic deformation, elastic limit, proportional limit, and yield strength affect materials • Describe the different types of stress in relation to materials • Explain the differences between resilience and toughness

4. Physical Properties • Based on Laws of Physics • Mass • Energy • Force • Light • Heat • Electricity • Other physical phenomena

5. Density • Mass of a material in a given volume • Common value is grams/cubic centimeter • Depends on the type of atoms present • High density metals feel heavy • High atomic numbers • Atoms closely packed

6. Density • In the example to the right, both cans have the same volume, but the classic coke is more dense because the sugar weighs more than nutrasweet. The density of classic coke is 1.11 g/ml, and the density of diet coke is 1.00 g/ml http://www.elmhurst.edu/~chm/vchembook/121Adensitycoke.html

7. Properties of Materials • Physical Properties • Based on laws of physics • Mechanical Properties • Material’s ability to resist forces • Chemical Properties • Setting reactions, setting & degradation • Biologic Properties • Effects on living tissues

8. Thermal & Electrical Properties • Materials that conduct electricity need to have insulation from the pulp • Electrical current generation • Usually by means of different metals in contact with each other (Galvanism) • Saliva facilitates flow of electrons between metals, producing an electrical current like a battery • Pain reaction to electrical current in tooth with deep filling (little insulating dentin)

9. Pulp Insulation • Normally, remaining dentin in a cavity preparation insulates the pulp. • When little dentin remains (within 1 mm of pulp), cement bases can be used to insulate pulp. • Composite and ceramic restorations are nonconductive and do not need insulators.

10. Boiling and Melting Points • Help identify chemicals • Mixtures have boiling range rather than a specific boiling point • Atomic bonds broken by thermal energy • Some materials don’t melt or boil… • Decompose (burn) – wood, cookie dough

11. Vapor Pressure • Measure of tendency to evaporate • Higher temperature increases vapor pressure • Molecules escape from liquid to form gas • Useful as solvents • Solvent evaporates, leaving a film of desired material (Copal varnish, etc.)

12. Thermal Conductivity • The rate that heat flows through a material • Metals have low heat capacity • Readily warms up and transmits heat • Example: Temperature change of hot food (55°C) and pulp (37°C) provides strong stimulus • Insulating base of .75 – 1mm minimize effects of rapid temperature change

13. Thermal Conductivity • Measurement depends on: • Distance the heat travels • Difference in temperature between source and destination (water pipe) • Measured in heat flow over time • Calories/second·meter·degree • Insulating material needed to protect pulp with deep metal restoration

14. Thermal Conductivity • Measure of heat transfer • Rate of heat flow

15. Heat Capacity • Amount of energy it takes to raise the temperature of that object 1° • Specific heat capacity is the amount of energy it takes to raise the temperature of 1 unit of mass of that material 1°

16. Heat of Fusion & Vaporization • Amount of energy needed to melt a material = heat of fusion • Need 80 times more energy to melt ice than to raise the temperature of water 1° • Amount of energy needed to boil a material = heat of vaporization • Need 540 times the energy to boil the same quantity of water

17. Coefficient of Thermal Expansion • Measurement of change in volume in relation to a change in temperature • Cooling results in shrinkage/contraction • Compare dental material to tooth • Restoration will shrink with cold and expand with heat • Opens gaps between restoration and tooth = microleakage (may cause recurrent decay) • Opening and closing gap = percolation Dental amalgam – percolation decreases over time due to corrosion products from the amalgam filling the space

18. Thermal Properties

19. Electrical Conductivity • Metals are good conductors • Polymers and ceramics are poor conductors – insulators • Affects corrosion of metals Electric pulp testing – need to know what material is in or on the tooth

20. Viscosity • Ability to flow • Measured in grams/meter·second, or poise (P) • Temperature-dependent property • Thick = flow poorly (cold syrup) • Thin = flow easily (warm syrup) • Water at 20ºC = 0.01 P (1 cP) • Impression materials between 100,000 and 1,000,000 cP

21. Wetting • Low viscosity and ability to wet a surface are important in dental materials • Measured by determining the contact angle of a liquid or solid Low contact angle = good wetting • Example: drop of water on ice cube High contact angle = poor wetting • Example: drop of water on plastic

22. Wetting • Partial wetting-contact angle around 90 degrees • Non wetting-close to 180 degree contact angle http://en.wikipedia.org/wiki/Contact_angle

23. Hardness • Measured by pressing a hard shaped tip into the surface of a material • Brinell, Rockwell, Vickers, Knoop • Calculated based on: • Size of indentation • Load on the tip • Shape of the tip • Knoop (KHN): enamel = 350, dentin = 70, porcelain = 400-500, acrylic denture teeth = 20

24. Abrasion Resistance • Goldilock’s Principle (Just Right!) • Wear resistance of dental materials to food and opposing teeth • Hard enough to wear well, but not wear away opposing teeth

25. Solubility • Calculated by amount of material that dissolves in a given amount of liquid in a given time • Test by immersing in water • Sample weighed before and after • Weight difference is solubility • Dental material should be nearly zero

26. Water Sorption • Ability to absorb water • Measured much like solubility Weight gained is the water sorption

27. Color • Complex phenomenon • Psychologic response to a physical stimulus • Perception of color may differ between people • Color depends on light (hard to match restorative material to adjacent teeth) • Measured by matching against color tabs • Spectrophotometer (not useful in clinical dentistry) • Fluorescence is important • Color of teeth is in the yellow range

28. Interaction with X-Rays • Some materials are radiolucent • Not seen in XR • Radiopaque – metals • Some materials match radiopacity of enamel to allow diagnosis of recurrent caries – makes them hard to detect on XR, though!

29. Mechanical Properties • Subgroup of physical properties • Describe a material’s ability to resist forces • Elasticity, stress, strain

30. Biting Forces Force: Any push or pull upon matter Stress: The reaction within the material to an externally applied force Strain: The change produced within the material as the result of stress

31. Types of Forces Compression – pushing or crushing stress • Average biting force in posterior is ~170 lbs. or about 28,000 psi on a single cusp of a molar

32. Forces • Tension – pulling stress (tug of war) • Shear – parts of an object slide by each other • Torsion – twisting force • Bending – combination of several types of stresses • One side stretched, other side compressed ↑ ↓

33. Forces in single dimensions • Compression • Tension • Shear • Torsion • Flexure • Diametrical tension/compression This video also available separately on ANGEL

34. Stress and strain • When force is applied to an object, it deforms • Stress-load per unit of cross-sectional area (eg. pounds per square inch); the resistance a material makes to an applied load • Strain-deformation per unit length; the change in shape (deformation) a material makes in response to stress

35. Stress and strain • If a pile of books is placed on a shelf, the weight of the books exerts a downward force on the shelf. The shelf does not fall down, the shelf resists the weight of the books. This resisting force is stress. If the shelf were to change shape (eg. sag in the middle) as a result of the weight of the books, the amount of change would be the strain. http://www.shelvingcompany.co.uk/media/heavy-duty%20floating_shelf.jpg

36. Strain & Stress • Strain : Change in length divided by the original length • Fractions (0.02) or percent (2%) • Stress : Force that develops in loaded object (load) • Stress = load/area • Pounds/square inch (psi) Stress and strain are proportional

37. Elasticity • When force is removed, the object returns to its original shape • Atomic bonds = microscopic springs • Bending = stretching + compression of atomic bonds • Compression or elongation of a loaded object – measured in terms of change in length

38. Young’s modulus • Modulus of elasticity • Measure of the material’s rigidity or stiffness • Resistance of the material to strain or deformation • High modulus = stiff material (enamel) • Low modulus = more flexible (rubber band) • Units are psi, but larger (psi x 106 or gigapascals)

39. Strain • Elastic strain: deformation/strain that is reversible (eg. stretching an elastic band a little and it bounces back to its original shape and size) • Plastic strain: some permanent deformation caused (eg. stretching an elastic band really wide to the point that when it relaxes, it remains a little stretched out) • The tipping point between elastic and plastic strain is the elastic limit

40. Plastic Deformation • Stress no longer proportional to strain • Spring doesn’t return to original length • Elastic limit, proportional limit, yield point • Ultimate tensile strength – point where material breaks (failure occurs) • Highest stress on the graph • Bad for bridges – road or dental ones • Ultimate strength = highest stress measured • Compressive test shows compressive (tensile) strength

41. Mechanical Properties of Dental Materials • Resilience– ability to absorb energy and not be deformed (mouthguard) • Toughness– energy absorbed up to the failure point on stress/strain diagram (helmet) • Fracture toughness – energy it takes to fracture a material when a crack is present • Glass = low Metals = high

42. Fatigue • Fatigue – testing replicates real world applications • Materials used multiple times – things fail eventually • Testing predicts amount of stress the material can endure without breaking

43. Time-dependent Properties • Creep – very slow flow • Small change in shape when an object is under continuous compression (amalgam) • Takes place over a long period of time • Temperature dependent • Stress relaxation – similar to creep • Slow decrease in force over time (ortho elastics) • Temperature dependent

44. Stress Concentration • Stress focuses around defect • Glass cutter scratches surface (defect) • Bending stress applied; fracture occurs • Control the defects – it’s important in dentistry to handle materials properly • Remove surface defects that concentrate stress • Polish restorations, proper design, glaze porcelain

45. Chemical Properties • Decay or degradation • Setting Reactions • Gypsum products set by precipitation • Composites polymerize

46. Biologic Properties • Effects of a material on living tissue